M500 minimum speed well below guaranteed minimum?

I've recently upgraded (around 1 month ago) to the M500 broadband package hoping it would improve the stability and speed of my broadband internet.

Since getting it, the highest download speed I've had from a hardwired connection is around 57Mb, well below the guaranteed minimum of 258Mb.

I've tried contacted Virgin Media on the telephone to no avail and they're online support of resetting the hub isn't much help either! 

I have checked things from my end and unplugged by wireless router and checked my network cable, ethernet card, etc, and have found slight improvement by removing the router i have, however still only going to around 87Mbps.

I suspect that the issue lies with either the inlet on my wall outside, one of the coax cables or maybe the cabinet as nothing has been touched since it was installed somewhat 14 years or so ago.  

Does anyone know what can I do if I consistently don't get the minimum guaranteed 258Mb download speed?